This invention relates generally to computer disk drive units and related control systems for use in a personal computer environment or the like. More particularly, this invention relates to a system capable of periodically monitoring and compensating for thermal offset of one or more data read/write heads relative to a servo head in a high density disk drive unit, in a manner minimizing both read/write error and functional time requirements for the compensation calculation and application.
In recent years microcomputer equipment such as personal or desk top computers have become extremely popular for a wide range of business, educational and recreational uses. Such computers typically include a main central processor having one or more memory storage disks for storage of data. The storage disk or disks are commonly provided as part of a so-called Winchester disk drive unit, sometimes referred to as a "hard" disk, wherein the storage disks are rotatably supported within a substantially sealed housing. The disks are rotatably driven in unison by a small spindle motor, and one or more electromagnetic heads are displaced by a head actuator assembly to traverse surfaces of the disks for purposes of reading and writing data. Such data is recorded onto magnetizable surfaces or surface films of the disks in the form of data bits located within narrow, closely spaced concentric tracks on the disks. Accordingly, for any given disk drive unit the total memory storage capacity is directly proportional to the number of disks as well as the number of tracks on each disk.
In normal operation, a system controller of the main central processor has the capability to identify data stored on the disks, typically through the use of directory name, file name, and/or track address information. When reading of data at a specified data track is desired, the system controller displaces the head or heads to the desired position by supplying appropriate command signals to operate the head actuator assembly. Alternatively, when data recording or writing is desired, the system controller operates the actuator assembly to align a selected head within a vacant data track. In either case, the head actuator assembly is commanded to displace the head or heads through a generally radial path relative to the disk for moving each head from alignment with a previously selected track to alignment with the newly selected destination track. Such movement of the head is normally referred to as a "seek" step.
In one known servo seeking and tracking system, each surface of each disk includes servo information prerecorded within radially spaced servo segments or sectors. This type of servo system, commonly known as an "embedded" servo system, provides data which is read by the operating head for purposes of seeking and tracking within a designated data track. During head traverses to seek a new specified data track, the head responds to embedded track address information within the servo sectors to identify a specified destination track. While such embedded servo systems have functioned generally satisfactorily, such systems inherently include track seek or access speed limitations due primarily to the limited sample rate at which the head can read and respond to servo information within the servo sectors. This inherent seek speed limitation is rendered more significant when relatively high track densities are used. Moreover, an embedded servo system fixes the number of available data and servo sectors in a manner which is incompatible with some types of controller surfaces.
So-called "dedicated" servo surface systems are also generally known in the art, wherein servo seeking and tracking information is prerecorded onto one surface of a memory storage disk in a disk drive unit having multiple disks. The servo seeking and tracking information is detected by an associated servo head for purposes of displacing and aligning a group of data heads with a designated data track, or to identify track crossings during a seek/access step. Such dedicated servo surface systems are preferred in certain multiple disk microcomputer applications since they can provide an overall increased data storage capacity to the disk drive unit.
For optimum disk drive unit performance it is desirable to displace the head or heads through each seek step in a minimum time period consistent with accurate head alignment with the destination track. In dedicated servo surface systems each of the data heads are displaced coextensively with the servo head for alignment with a designated "cylinder" address (a cylinder address being the alignment of all data heads over a selected track on the designated servo surface). Data applied to a particular platter or disk surface by any one of the data heads may then be indexed at a particular cylinder address. However, track seek/access errors can arise as a result of thermal expansion of the actuator arms and other mechanical components of the hard disk drive unit, resulting in a misalignment of the data heads relative to the servo head. In some cases, thermal expansion of the mechanical parts may cause one or more data heads to displace from the designated cylinder address sufficiently to introduce read/write error.
There exists, therefore, a significant need for a system capable of compensating for thermal offset of data heads relative to the servo head, which system is relatively simple and operates automatically to minimize thermal expansion-induced read/write error. Further, such a thermal offset compensation system is needed which is specifically designed for use in dedicated servo surface hard disk drives, and which minimally impacts the speed efficiency of the computing system. Moreover, a thermal offset compensation system is needed which is capable of calculating an initial compensation offset value upon spin-up of a disk drive unit, and thereafter periodically monitoring changes in the thermal expansion-induced offset. The present invention fulfills these needs and provides further related advantages.